As recording densities beyond 100 Gb per sq. in. and track widths below 0.1 microns become feasible, the traditional CIP (current in film plane) spin valve head can no longer meet the required signal level because the signal amplitude of a CIP head scales with track width. CPP (current perpendicular to plane) heads then become potential candidates. CPP mode has many advantages over the CIP mode, such as higher GMR ratio (by using multilayers), good joule heat dissipation (conductor as gap material), and a signal amplitude that is independent of track width.
Pillar-type spin valve CPP heads have been built and tested [1,2]. In the pillar-type design, a contiguous hard bias scheme is needed to stabilize the free layer edge domain. However, this reduces the head sensitivity dramatically for track widths below 0.1 microns, just as in the CIP spin valve abutted junction design. It is therefore not suitable for densities beyond 100 Gb psi.
A modified pillar-type design has been proposed [3] and is shown in FIG. 1. Shield 19 sits atop antiferromagnetic layer 18 which serves to pin synthetic antiferromagnetic trilayer 15/16/17. Copper spacer layer 14 is immediately below. In this design, continuous free layer 13 remains but is exchange coupled with antiferromagnetic film 12. Micro-magnetic simulation shows that the free layer sensitivity drops only about 10% for a pinning field of about 150 Oe [3]. This sensitivity is much higher than for the abutted junction case. However, in the CPP mode, the current-induced magnetic field has a circular direction within the GMR film plane. This circular field yields a buckling domain in the free layer which may cause instability and noise during head operation.
Another proposed design is the synthetic pattern exchange structure [4] seen in FIG. 2. In this scheme, the entire GMR stack, which includes free layer 13, Cu spacer layer 14, AFM pinned AP1 layer 15, and AP2 layer 17 (layer 16 being ruthenium), all extend outward to a significant extent. The track width is defined by conductor 25 at the center of the stack. The sides of the free layer are pinned by a synthetic pattern exchange layer 24/23/22. The advantage of this design is its high sensitivity and good track width definition. Due to the strong synthetic pinning strengths of the free layer side regions, the effects of circular field on the free layer can be largely eliminated. Layer 26 is the top shield.
However, this design also has several drawbacks:                (1) the extended Cu spacer layer 14 causes current leakage which results in signal loss.        
(2) the large shape anisotropy in the AP layer due to the large aspect ratio results in significant deviation of the AP layer from a transverse direction; strong AP coupling is needed for this design at small dimensions.
(3) there is difficulty with bias point control. In the CIP spin valve head, there are two magnetic fields along the transverse direction (one is the stray field from the AP layers and the other is a current induced field) which counterbalance each other. In the CPP mode, however, the current field in this direction vanishes and this stray field becomes the only transverse field.